The invention relates to a system for the preparation of compressed air having a heat exchanger (system heat exchanger) which is in communication with a pressure source on the inlet side on the compressed air side and which is in communication with a system or unit on the outlet side on the compressed air side which is to be supplied with cooled compressed air and comprising at least one heat exchanger (air conditioning system heat exchanger) which is in communication with a pressure source on the inlet side on the compressed air side and which is in communication with further components of an aircraft air conditioning system on the outlet side on the compressed air side.
In a preferred aspect, the invention relates to an architecture for the preparation of compressed air which is required for the operation (supply) of oxygen and nitrogen generating units in aircraft.
With passenger aircraft, it is necessary to control the temperature of the cabin (cool/heat) and to ventilate and pressurize it. Different systems are required for this such as:
a) Air Conditioning Plants
                The air coming from the compressed air bleed system is cooled in the air conditioning system in accordance with the arising cooling and heating demands in order to ensure a constant temperature in the cabin. In addition, the required fresh air throughput to ventilate the cabin is ensured.b) Cabin Pressure Control System        This system regulates the cabin pressure in that more or less cabin air flows back to the environment by means of a variable valve position.        
The failure of one of these systems or a high leakage of the cabin results in a drop in cabin pressure. Furthermore, heavy smoke development in the cabin can occur due to different malfunctions. In all these cases, an emergency oxygen supply is required for the passengers and the aircraft crew.
It is known to ensure the emergency oxygen supply by an emergency oxygen system. For this purpose, oxygen flasks under pressure are carried along in the aircraft which dispense oxygen to the passengers and to the aircraft crew by means of masks in an emergency. This emergency oxygen system is, however, associated with various disadvantages such as weight, limited oxygen quantity, space requirements, risk of explosion and time-consuming and expensive inspection and refilling.
In the meantime, alternative systems are available for the emergency supply of oxygen. It is known, for example, from DE 41 04 007 A1 and EP 1 375 349 A1 to press cooled compressed air through one or more molecular sieve concentration apparatuses (OBOGS=on board oxygen generating system). A fresh air/product gas having an oxygen content of up to approximately 95% thereby results at the OBOGS outlet. In a case of emergency, this air is then directed directly to the oxygen masks and/or for the refilling of the now considerably reduced oxygen flasks.
There are different OBOGS molecular sieve filler materials in this process. It is, however, common to all these materials that the molecular sieve apparatuses have to be supplied with cooled compressed air from approximately 0° C. to 60° C. and a relative pressure from approximately 1.4 bar (rel) to 4.0 bar (rel). In accordance with the prior art, the OBOGS supply takes place in that hot compressed air (approx. 200° C.) is directed from the compressed air bleed system into a special OBOGS heat exchanger. In this process, the air is cooled to the required temperature range of approximately 0° C. to 60° C., as can be seen from FIG. 1.
The OBOGS is very rarely in operation since the probability of a failure of the cabin pressurization is very low due to the integrated redundancies.
In addition to the aforesaid system for the emergency oxygen supply, a system is known for the generating of nitrogen which will be described in more detail in the following:
The aircraft tanks are filled with a mixture of kerosene and air due to the removal of kerosene during the flight. On an unfavorable composition, an explosive mixture can arise which ignites itself or due to spark formation.
Studies have shown that a spontaneous ignition of the mixture can be reliably avoided by a reduction in the (normal) oxygen content of the air in the kerosene tank from 21% to approximately 12% to 14.5%. In addition to a low oxygen content, a high nitrogen content is advantageous to prevent spontaneous ignition.
A unit is described in EP 1 375 349 A1 for the generating of nitrogen which is termed an OBIGGS (on board inert gas generating system). In a comparable manner to the OBOGS, this OBIGGS is also based on molecular sieve technology, with here, however, oxygen being filtered. A product gas having a much reduced oxygen content (less than approximately 10%) and a substantially increased nitrogen content arises at the outlet of the OBIGGS. This product gas can now be directed into the kerosene tank to avoid the risk of a spontaneous ignition of the kerosene/air mixture. The OBIGGS product gas can furthermore e.g. also be directed into freight spaces to minimize the fire hazard.
Analog to the OBOGS, the OBIGGS requires a supply with cooled compressed air from approximately 50° C. to 90° C. Depending on the filter technology used, the required supply pressure amounts to approximately 1.7 bar (rel) to 6.0 bar (rel) for PSA (=pressure swing absorption process) filters and to approximately 1.4 bar (rel) to 4.1 bar (rel) for HFM (=hollow fiber membrane) filters. In accordance with the prior art, the OBIGGS supply takes place as with the OBOGS in that hot compressed air (approximately 200° C.) is directed from the compressed air bleed system into a special OBOGS/OBIGGS heat exchanger and is cooled there in accordance with the demands.
Such a system is shown in FIG. 1. FIG. 1 shows that the heat exchanger OHX, which is in communication with the OBIGGS/OBOGS and which is termed a system heat exchanger in the following, is charged with compressed air (bled air) from the engines on the inlet side. The inlet line can be closed by means of the valve OSOV. A control valve OBPV is furthermore shown by means of which a bypass line around the system heat exchanger OHX can be closed. The reference symbols OESOV and OEJ are components of a jet pump which is required to convey ambient air through the system heat exchanger OHX in ground operation.
As can further be seen from FIG. 1, the system heat exchanger OHX is cooled by means of ram air or by means of ambient air in ground operation.
The cooled compressed air is available on the compressed air side of the outlet side of the system heat exchanger and is then supplied to the OBIGGS/OBOGS. The desired product gas can be removed or supplied to the destinations in question on the outlet side of said OBIGGS/OBOGS.
A high demand on the availability of the compressed air supply can also have the result that the architecture shown in FIG. 1 is present in duplicate in a parallel arrangement.
In contrast to the OBOGS, the OBIGGS is predominantly in operation.
The embodiment for the cooling of compressed air described above is associated with various disadvantages:    a) An independent ram air passage, including the corresponding openings for inlet and outlet in the aircraft structure, is required for the ram air of the system heat exchanger OHX. The available space for freight is thereby reduced and the aircraft weight is increased and additional costs are generated.    b) For reasons of cost and complexity, a flap at the outlet of the ram air passage is generally dispensed with. There is thus always a ram air passage throughput during flight, even if the OBOGS and/or OBIGGS is switched off. The ram air throughput increases the aircraft kerosene consumption and thus the operating costs.    c) OBOGS and/or OBIGGS operation can also be necessary on the ground, in particular for military applications. Since no ram air is available in this operating state due to the lack of ram pressure, it must be actively conveyed. For this purpose, either a fan or a jet pump as shown in FIG. 1 (OEJ and OESOV) must be installed. Both versions are associated with disadvantages of cost and weight.